The tissues in the human oral cavity are generally exposed to repeated low doses of known or suspected carcinogens, such as alcohol, tobacco smoke, and x-radiation from diagnostic medical and dental radiology. The proposed study would examine the effects of concurrent, repeated exposures to low doses of x-radiation on tumor induction by repeated, low dose applications of a chemical carcinogen (DMBA), in established carcinogenic amounts, on an in vivo mammalian oral epithelial cell renewal system. Previous synergistic cocarcinogenesis studies of this type have been limited to relatively high radiation levels, and estimates of effects from low radiation doses have been generally arrived at through extrapolation from high dose data. Young adult Syrian golden hamsters will receive thrice weekly topical applications of 0.1% DMBA in mineral oil on the right cheek pouch; 1R or 3R x-irradiation thrice weekly to the right head and neck; or, both chemical and radiation treatments thrice weekly. Treatments shall continue until tumors have been induced in a preponderance of animals within a specific treatment group. The parameters of evaluation include the following: animal mortality; percentage tumor induction; tumor induction latent period; tumor volume and histopathology; and, tumor cytokinetic behavior. It is anticipated that the results of these experiments will provide information leading to some clarification of mechanisms involved in the interaction between concurrent exposures to low levels of x-radiation and chemical carcinogens, as well as provide a useful model system and preliminary data for more detailed quantitative studies of low level exposures to multiple carcinogens and factors which modify host responses to these exposures.